1. Field of the Invention
The invention resides in the field of telecommunication, and more specifically, it relates to a method of optimizing the search for cells in a mobile telecommunication network.
The invention also relates to a mobile device comprising means for optimizing the search for cells in a mobile telecommunication network.
More particularly, the invention finds its application in a telecommunication network comprising a plurality of cells, each cell accommodating a base station that exchanges synchronization data with a mobile device UE via a channel SCH to allow the mobile devices to perform measurements on at least one cell adjacent to the current cell so as to retrieve a scrambling code specific to the adjacent cell. In the telecommunication network, the synchronization data comprises an integral number of frames, each frame comprising an integral number of slots, and the channel SCH being subdivided into a primary synchronization channel used for detecting the beginning of a slot and a secondary channel used for detecting the beginning of a frame.
The invention finds its application in the search of cells by a dual mode GSM (Global System for Mobile Communications)/UMTS (Universal Mobile Telecommunications System) or single mode UMTS mobile telephone to prepare for a possible or contingent handover applying the procedure of cell search.
2. Description of the Related Art
To prepare for possible or contingent handovers, a mobile device UE engaging in communication has to identify neighboring cells. To do so, the mobile device applies a procedure of searching for cells that includes performing measurements on neighboring cells in order to switch to the cell that provides an optimal communication quality. This procedure may be implemented either in standby mode (watching mode) or in online mode (connected mode).
FIG. 1 schematically shows the case where the mobile device applies the procedure of searching for cells continuously in standby mode or at the time of initial activation. In this case, the procedure is initialized at time T1 and the processing is performed for a period D1 corresponding to an integral number N1 of time slots. At time T2 corresponding to the end of the period D1, the mobile device has the information necessary for carrying out the handover. Since all the processing is accomplished within the same time window 2 of a relatively short duration, and since the information collected within one time window is not reused in the following time window, a possible drift of the reception sampling clock does not have any effect on the performance of synchronization. The above processing is repeated in the same way in the following time window, also of a short duration D2.
FIG. 2 schematically shows the case where the mobile device applies the procedure of searching for cells in online mode (connected mode) intermittently. Contrary to the previous case, the mobile device utilizes several separate time windows 4, and combines the measurements collected in each time window 4 in a way to satisfy a given performance criterion depending specifically on the step of the cell search procedure at issue. Each time window allows a profile to be calculated and stored in memory, and at the end of each step, a specific correlation profile is calculated. In this case, the information and results from the processing of a given time window are reused in the processing performed in the following time window. Therefore, the processing time may be relatively long compared to the processing time in the continuous mode, and the effect of a drift or shift of the clock of the mobile device is more marked than with the previous case.
FIG. 3 shows an exemplary correlation profile obtained for a conventional cell search in a UMTS network. On the horizontal axe of this profile, the beginning point of a slot corresponds to the maximum correlation between a signal received by the mobile device and the primary synchronization code (PSCH). The points of maximum correlation of the elementary profiles obtained in different time windows may vary when there is a clock drift. A major problem that arises when the sampling clock of the mobile device is imprecise and/or when there is a Doppler shift between the base station with which the mobile device is communicating and the cell that the mobile should identify may be explained as follows: If windows are 120 ms apart, and if the clock performs sampling at a frequency equal to (1+ε)*4*3.84 MHz, where ε=0.5×10−6, rather than 4*3.84 MHz, the points of the maximum correlation of two successive windows will be different by two peaks rather than one. This results in an ambiguity that degrades the performance of the synchronization slot and may cause the synchronization frame to fail.
Given that slot synchronization points detected in successive measurement windows are different, the correlation profile obtained by simply adding together correlation profiles associated with different measurement windows will present several peaks corresponding to different points. This is because the values of the peaks associated with different measurement windows are not added up, and the contrast or difference between the correlation peak and the threshold value representing correlation values not corresponding to the beginning of a slot is not improved.
Consequently, given that points corresponding to correlation peaks vary and they are of statistically equal values, the probability of detecting the beginning point of a slot corresponding to that obtained in the last window is small. This is because the detection is performed in a conventional manner by searching for the maximum of the average profile.
Also, the combination by simply adding together measurements with respect to different windows may lead to a global failure of the procedure because synchronization points with respect to different windows may vary.
The object of the invention is to solve this problem.